The invention relates generally to the field of power control systems. More specifically, the invention relates to techniques for maintaining a charge in a self powered gate driver system during periods of inactivity.
Generally, motor controllers, and more generally, electrical controllers operate by limiting the power from a power supply that reaches a motor or other load. Some motor controllers limit the power delivered to a motor by intermittently conducting current between the power supply and the motor. These motor controllers typically couple to a sinusoidal ac power supply and conduct current during a portion of each cycle of the sinusoid. Correspondingly, to limit the power delivered to the motor, the motor controller may not conduct during a portion of each cycle. Typically, the duration of the period during which the motor controller does not conduct current is adjustable. Consequently, by adjusting the duration of this non-conductive period, the operation of the motor or some other load may be controlled.
Some motor controllers selectively transmit power by conducting current through a pair of silicon controlled rectifiers (SCRs). As is well known in the art, an SCR is a type of solid state switch that includes a rectifier controlled by a gate signal. Thus, when turned on by the gate signal, the SCR permit current to flow from its anode to its cathode but not in the reverse direction. Once turned on, the SCR typically remains on until the gate signal is removed and the current decreases to near zero. In the off state, the SCR usually does not conduct current in either direction.
By applying a gate signal at the appropriate times, a motor controller may regulate the power delivered to a load. Typically, a motor controller will employ a pair of SCRs (for each power phase) connected between the power supply and load in an inverse parallel relationship. Thus, when regulating an alternating current power supply, one SCR will be forward biased during the positive portion of the voltage/current cycle, and the other SCR will be forward biased during the negative portion of the voltage/current cycle. The motor controller may conduct current between power supply and the load by applying a gate signal to whichever SCR is forward biased. Similarly, the motor controller may prevent current from flowing by not turning on the forward biased SCR or by turning on the forward biased SCR at some time after it becomes forward biased. Thus, the more time during a cycle that an SCR is both turned off and forward biased, the less time current may pass between the power source and motor, as the reverse biased SCR will not conduct current. Consequently, the operation of the load may be controlled by increasing or decreasing the time between when an SCR becomes forward biased and when it is turned on in each cycle.
Typically, a motor controller includes circuitry for turning on the SCRs. A motor controller may include a driver that delivers a small current to the gate electrode of an SCR. The driver may time the pulse of current to the gate electrode to regulate the power delivered to the motor. To deliver more power, the driver will delay for less time after a SCR becomes forward biased before turning on the SCR. Similarly, to reduce the power delivered to the motor, the driver will delay a longer period after a SCR becomes forward biased before turning on the SCR and permitting current to pass. Each driver includes circuitry for determining when to turn on the SCR. Typically, motor controllers employ one driver for each SCR. Thus, a motor controller regulating a single phase of ac power typically employs two drivers, whereas a three phase controller includes six.
Powering the operation of the drivers presents challenges. Often, the drivers connect to an SCR that is exposed to high voltages. For example, SCRs often connect to power supplies that operate at 2300 volts or higher. Thus, it may be important to keep the driver electrically isolated from other parts of the system. Some systems employ a single power supply for each driver. However, dedicated power supplies for each driver may add to system costs, the size of the system, and the number of components that may fail. Moreover, motor controllers often employ a large number of drivers. For example, as noted above, a motor controller that regulates power from a three phase ac power supply may employ six drivers, one for each of the two SCRs for each phase. Similarly, to regulate the power from higher voltage power supplies, a motor controller may employ two or more pairs of SCRs for each phase. Thus, a three phase system with three SCR pairs for each phase may employ 18 SCRs and 18 drivers. Consequently, powering each driver with a dedicated power supply becomes less desirable as the number of switches and drivers increases.
With some success, designers turned to self powered gate driver systems (SPGDSs) to avoid these issues. Typically, SPGDSs capture energy from the power supply driving the load (i.e., line power). Often, within an SPGDS, a series of capacitors connect to self powered circuitry that charges the capacitors. The SPGDS may exploit voltage differentials across the SCRs to draw current and store a charge. The charge on the capacitors can then be used to power the drivers. The voltage differentials exploited to charge the capacitors typically occur during the operation of the SCRs. As the SCRs intermittently conduct current between the power source and load, a voltage differential may form across the SCRs. Advantageously, a self powered system may avoid the isolation issues associated with dedicated power supplies for each driver. Also, the cost of the components directed toward powering the drivers may be lower in a self powered system than in a system employing dedicated power supplies for each driver.
However, SPGDSs are in need of improvement. When transitioning from certain modes of operation, the capacitors may lack enough charge to power the drivers. Consequently, a re-activated SPGDS may exhibit a delay before conducting current as the capacitors charge over a few cycles. For example, during full speed operation, some systems bypass the SCRs to deliver power directly from the power supply to the motor. Without a voltage differential across the SCRs, the capacitors may discharge, leaving the drivers without a source of power. Thus, when such a system transitions from full speed operation to a mode where the SPGDS limits the power delivered to the motor, the capacitors powering the driver may lack sufficient charge to restart the driver. The SCRs may remain off for a number of cycles while the capacitors build up a charge sufficient to power the drivers. Similarly, during system startup, the capacitors may have discharged due to a lack of potential across the SCRs. Consequently, during a transition from one of these periods of inactivity, the system may not respond for a few cycles as the capacitors recharge. During this delay, the SCRs may remain off, resulting in an abrupt transition in the power reaching the motor. These sudden transitions may lead to current and torque surges that could contribute to wear on various components within the system.
There is a need in the art for a system that maintains a charge in these SPGDSs during periods of inactivity. There is a particular need for accomplishing this without incurring the costs associated with individual power supplies coupled to each driver.